The present invention relates to systems and methods for thermally processing a wafer-like object through a temperature profile that preferably includes both heating and cooling the wafer-like object. In particular, the present invention is directed to an apparatus having a thermal processing chamber which can support, heat and cool such an object with thermal uniformity and effective heat transfer even when the heating requirements are high. The present invention also allows the creation and maintenance of a processing environment constituted of precisely controlled mixtures of gases that may vary widely from the ambient environment.
The present invention has been developed for its particular applicability in the processing of semiconductor wafers, such as for making microelectronic devices, where such processing requires precise temperature control and temperature changes. This processing may also require control of the gas mixtures allowed to contact the wafer during the process. Many other types of products and processes involve thermal processing with accurate temperature control of an object, such objects hereinafter referred to as xe2x80x9cwafer-likexe2x80x9d objects.
In the manufacture of microelectronic devices, such as integrated circuits, flat panel displays, thin film heads, and the like, processing often involves the application of a layer of some material, such as a dielectric, onto the surface of a substrate, such as a semiconductor wafer in the case of integrated circuits. Dielectrics, for example, may need to be baked and then cooled to cure. To prevent oxidation of such a dielectric material, for example, after any processing there of by a baking step, the wafer must be cooled to a certain temperature in an environment of reduced oxygen (an anaerobic environment). Cooling of the wafer also reduces the risk of thermal damage to the wafer transfer mechanism during wafer transfer after processing. The baking and cooling steps must be precisely controlled within exacting temperature constraints to ensure that the selected portions of the dielectric properly set with its desired material properties. Baking and cooling operations for microelectronic devices typically involves cycling a wafer-like object through a desired temperature profile in which the object is maintained at an elevated equilibrium temperature in a controlled environment, cooled to a relatively cool equilibrium temperature, and/or subjected to temperature changes of varying rates (in terms of xc2x0 C./s) between the equilibrium temperatures. To accomplish baking and cooling, previously known bake/chill operations have included separate bake and chill plates that have required the use of a wafer transport mechanism in order to physically lift and transfer the wafer itself from one place to the other. This approach presents a number of drawbacks. First, wafer temperature is not controlled during transfer between the bake and chill plates. Second, the overall time required to complete the bake/chill process cannot be precisely controlled because of the variable time required to move the wafer to and from the respective plates. Third, the required movement takes time and thus reduces the throughput of the manufacturing process. Fourth, the cost of equipment is higher than necessary because the apparatus requires extra components to handle the wafer during transport from plate to plate. Fifth, the mechanical move from plate to plate introduces the possibility of contaminating the wafer. Sixth, the wafer is exposed to atmospheric oxygen while it is at elevated temperatures, increasing the risk of oxidation. Seventh, the wafer transfer mechanism is exposed to elevated temperatures, reducing its reliability and/or increasing the complexity and expense of its design.
To overcome these deficiencies, a combination bake/chill apparatus has been previously developed by the assignee of the present invention, which is described in copending U.S. patent application Ser. No. 09/035,628, filed Mar. 5, 1998 and entitled xe2x80x9cCombination Bake/Chill Apparatus Incorporating Low Thermal Mass, Thermally Conductive Bakeplatexe2x80x9d, the entire disclosure of which is incorporated herein by reference. That combination bake/chill apparatus includes a low thermal mass, thermally conductive bakeplate to support a wafer during both its baking and chilling operations. With the wafer on one surface of the bakeplate, the other surface of the bakeplate is selectively brought into or out of thermal contact with a thermally massive chill plate so as to switch between baking and chilling operations. In one version, the bakeplate can rest on top of the chill plate during chilling, and one or both of the components is moved to separate them during baking. The bakeplate can heat a wafer by direct conduction of heat generated by the bakeplate to the wafer, while chilling requires heat transfer from the wafer through the bakeplate (which is not heated during the chilling operation) to the chill plate by conduction, which itself is preferably artificially cooled. Both the bake and chill plates are operatively supported within a housing defining a thermal processing chamber. In particular, the housing is formed as a cylinder comprising a cylindrical side wall, a flat top wall, and a flat bottom wall through which various control components extend. The side wall is split so that the top and bottom walls are relatively movable from one another to provide access within the process chamber for loading and unloading wafers.
In developing the present invention, it was discovered that thermal uniformity of a wafer-like object within such a processing chamber is significantly affected by the design and make-up of the process chamber itself. That is, the components making up the processing chamber as well as the components within the chamber, such as for supporting, heating and cooling a wafer-like object, significantly affect the temperature of the wafer-like object throughout its surface area. This is particularly true where such a wafer-like object is to be uniformly heated at relatively high temperatures, e.g., above 200xc2x0 C. and as high as 450xc2x0 C. or more. Newer polymers and coatings for semiconductor wafers cure at temperatures of between 350xc2x0 C. and 450xc2x0 C., for example. However, as noted above, precise temperature achievement of the entire surface area of a wafer-like object may be required for effective curing or processing. Such thermal uniformity being required in spite of the fact that such a processing chamber should advantageously be designed as a combination baking and cooling apparatus. That is, thermal uniformity is desired even where a wafer-like object is to be heated and cooled within the same chamber. Thus, the structure defining the process chamber and its internal devices not only affect the uniformity of the thermal processing that is conducted on a wafer-like object, they also are subject to cyclical heating and cooling. In general, thermal uniformity in processing a wafer-like object is a function of the relative thermal uniformity of the chamber and its components. So, to achieve good thermal uniformity, such as during a baking step, the process chamber housing and components should be together brought within a desired temperature range. But, as a result of a subsequent cooling operation, the entire chamber and components would be cooled, or at least its temperature uniformity would be compromised. In any case, cycle times would be lengthened in that the achievement of thermal uniformity of a next heating process would require greater time to assure a subsequent achievement of sufficient temperature uniformity of the process chamber.
In developing the present invention, it was also discovered that the gases contained within the processing environment of a baking and cooling apparatus during both steps should be controlled for enhancing the development of the desired material properties.
The present invention overcomes the deficiencies and shortcomings of the prior art by providing an apparatus and method for efficiently and effectively heating and cooling a wafer-like object within a controlled environment in the same process chamber. In accordance with the present invention, the process chamber and its components are designed to enhance thermal uniformity for the thermal processing operation, but to permit a temperature profile to be conducted including heating and cooling steps with maximized throughput through the apparatus. In particular, the process chamber can uniformly heat objects to high temperatures and still provide effective cooling in situ, all of which may advantageously occur in an environment where the mixture of gases can be carefully controlled.
In accordance with the present invention, good thermal uniformity can be achieved across the surface area of a wafer-like object while the wafer-like object can achieve sufficiently high and low temperatures in accordance with a desired temperature profile. In particular, the process chamber is designed so that its inner surface remains of a sufficiently high temperature relative to the desired temperature of the heating operation even during the cooling of the wafer-like object. Thus, during a subsequent heating step, good thermal uniformity can be achieved with respect to the surface area of a subsequent wafer-like object and with greater throughput. Preferably, the process chamber is also sufficiently sealable and closeable by a chamber door so that the thermal processing can be conducted within an anaerobic environment created by the suitable flow of inert gases as well.
The above advantages are achieved by carefully controlling the flow of heat (radiatively, convectively and conductively) as well as the composition of the gases in the processing environment. Radiative heat transfer control is achieved by maintaining the surfaces xe2x80x9cvisiblexe2x80x9d to the wafer-like object as close to the processing temperature as possible. Convective heat transfer control is achieved by establishing the proper gas flow pattern on the exposed surface of the wafer-like object. This flow pattern may be called upon to correct for other nonuniformities in the process. Conductive heat transfer control is achieved by ensuring uniform contact between the wafer-like object and the heating surface and by ensuring that the contacting surface temperature is as uniform as possible. Control of the gas composition in the processing environment is achieved by isolating the processing environment from the ambient environment and by maximizing the flexibility and control of gas flow in the processing environment.
In the present invention, the surfaces visible to the wafer-like object are preferably maintained close to the processing temperature through the use of three essential features. First, the chamber incorporates a double walled design that allows superior thermal isolation of the inner surface from the much cooler outer surface. Second, the cooling required to maintain some chamber seals within their thermal operating ranges is accomplished by the use of an internal gas cooling channels rather than a liquid cooling channel. Due to their generally lower heat capacity than liquids, gases allow more precise and reliable temperature control by permitting finer control of the heat transfer rate. The thermal limit of liquids also constrains their boiling point, which can create safety as well as reliability hazards. The thermal limit of gases allows the chamber wall to operate at higher temperatures, reducing heat transfer from the wafer-like object and, therefore, improving temperature uniformity. Third, careful control of the heat transfer from the heating element to the chamber bottom and side walls prevent temperature gradients along the walls, improving the uniformity of the visible surfaces. This control may be achieved by reducing the cross sectional area of conductive paths or by increasing their lengths. Control may also be achieved by minimizing emissivity of the heater surface thereby minimizing the radiation between the heater and the chamber wall.
The present invention promotes the development of the proper convective gas flow through the use of three elements. First, the use of separately variable inner and outer gas introduction patterns above the wafer-like object allow the ratio and magnitude of the flows to be adjusted to achieve the optimum flow pattern on the wafer-like object. Second, the use of a door minimizes gas disturbances during transfer of the wafer-like object, minimizing the time required to establish the required gas flow. Third, the use of removable exhaust plate simplifies the investigation of widely varying exhaust patterns, promoting the achievement of the optimum gas flow environment.
To ensure superior conductive heat transfer control, the present invention preferably employs a xe2x80x9cpedestalxe2x80x9d style heater that contacts the cooler chamber bottom wall at a single, preferably central point. This point of contact may then be carefully minimized to reduce losses to the chamber, improving pedestal surface uniformity. Heater surface uniformity is also improved by maximizing the radiative emissivity between the wafer-like object and the contacting heater surface while minimizing the radiative emissivity between the other heater surfaces and the chamber wall. The radiative emissivity of the heater surfaces may be controlled by chemical (e.g. anodization) or mechanical (e.g. ball peening) treatment. In particular, the surface or surfaces visible to the wafer-like object may be anodized while the other surfaces are left with a finely machined finish. To ensure good contact between the wafer-like object and the heater surface, channels on the pedestal surface are evacuated, the resulting pressure difference across the wafer-like object driving it against the heater surface.
To control the gas composition in the process environment, the present invention employs a door that, as was previously described, minimizes gas disturbances during transfer of the wafer-like object. The separate inner and outer gas introduction patterns allow sophisticated purging routines to be developed that can create the proper gas composition in the minimum amount of time. The removable exhaust plate assists in establishing the optimum gas flow pattern.